text: /*-------------------------------------------------------------------------
   *
   * walwriter.c
   *
   * The WAL writer background process is new as of Postgres 8.3.  It attempts
   * to keep regular backends from having to write out (and fsync) WAL pages.
   * Also, it guarantees that transaction commit records that weren't synced
   * to disk immediately upon commit (ie, were "asynchronously committed")
   * will reach disk within a knowable time --- which, as it happens, is at
   * most three times the wal_writer_delay cycle time.
   *
   * Note that as with the bgwriter for shared buffers, regular backends are
   * still empowered to issue WAL writes and fsyncs when the walwriter doesn't
   * keep up. This means that the WALWriter is not an essential process and
   * can shutdown quickly when requested.
   *
   * Because the walwriter's cycle is directly linked to the maximum delay
   * before async-commit transactions are guaranteed committed, it's probably
   * unwise to load additional functionality onto it.  For instance, if you've
   * got a yen to create xlog segments further in advance, that'd be better done
   * in bgwriter than in walwriter.
   *
   * The walwriter is started by the postmaster as soon as the startup subprocess
   * finishes.  It remains alive until the postmaster commands it to terminate.
   * Normal termination is by SIGTERM, which instructs the walwriter to exit(0).
   * Emergency termination is by SIGQUIT; like any backend, the walwriter will
   * simply abort and exit on SIGQUIT.
   *
   * If the walwriter exits unexpectedly, the postmaster treats that the same
   * as a backend crash: shared memory may be corrupted, so remaining backends
   * should be killed by SIGQUIT and then a recovery cycle started.
   *
   *
   * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
   *
   *
   * IDENTIFICATION
   *	  src/backend/postmaster/walwriter.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include <signal.h>
  #include <sys/time.h>
  #include <time.h>
  #include <unistd.h>
  
  #include "access/xlog.h"
  #include "libpq/pqsignal.h"
  #include "miscadmin.h"
  #include "postmaster/walwriter.h"
  #include "storage/bufmgr.h"
  #include "storage/ipc.h"
  #include "storage/lwlock.h"
  #include "storage/proc.h"
  #include "storage/smgr.h"
  #include "utils/guc.h"
  #include "utils/hsearch.h"
  #include "utils/memutils.h"
  #include "utils/resowner.h"
  
  
  /*
   * GUC parameters
   */
  int			WalWriterDelay = 200;
  
  /*
   * Number of do-nothing loops before lengthening the delay time, and the
   * multiplier to apply to WalWriterDelay when we do decide to hibernate.
   * (Perhaps these need to be configurable?)
   */
  #define LOOPS_UNTIL_HIBERNATE		50
  #define HIBERNATE_FACTOR			25
  
  /*
   * Flags set by interrupt handlers for later service in the main loop.
   */
  static volatile sig_atomic_t got_SIGHUP = false;
  static volatile sig_atomic_t shutdown_requested = false;
  
  /* Signal handlers */
  static void wal_quickdie(SIGNAL_ARGS);
  static void WalSigHupHandler(SIGNAL_ARGS);
  static void WalShutdownHandler(SIGNAL_ARGS);
  static void walwriter_sigusr1_handler(SIGNAL_ARGS);
  
  /*
   * Main entry point for walwriter process
   *
   * This is invoked from BootstrapMain, which has already created the basic
   * execution environment, but not enabled signals yet.
   */
  void
  WalWriterMain(void)
  {
  	sigjmp_buf	local_sigjmp_buf;
  	MemoryContext walwriter_context;
  	int			left_till_hibernate;
  	bool		hibernating;
  
  	/*
  	 * If possible, make this process a group leader, so that the postmaster
  	 * can signal any child processes too.	(walwriter probably never has any
  	 * child processes, but for consistency we make all postmaster child
  	 * processes do this.)
  	 */
  #ifdef HAVE_SETSID
  	if (setsid() < 0)
  		elog(FATAL, "setsid() failed: %m");
  #endif
  
  	/*
  	 * Properly accept or ignore signals the postmaster might send us
  	 *
  	 * We have no particular use for SIGINT at the moment, but seems
  	 * reasonable to treat like SIGTERM.
  	 */
  	pqsignal(SIGHUP, WalSigHupHandler); /* set flag to read config file */
  	pqsignal(SIGINT, WalShutdownHandler);		/* request shutdown */
  	pqsignal(SIGTERM, WalShutdownHandler);		/* request shutdown */
  	pqsignal(SIGQUIT, wal_quickdie);	/* hard crash time */
  	pqsignal(SIGALRM, SIG_IGN);
  	pqsignal(SIGPIPE, SIG_IGN);
  	pqsignal(SIGUSR1, walwriter_sigusr1_handler);
  	pqsignal(SIGUSR2, SIG_IGN); /* not used */
  
  	/*
  	 * Reset some signals that are accepted by postmaster but not here
  	 */
  	pqsignal(SIGCHLD, SIG_DFL);
  	pqsignal(SIGTTIN, SIG_DFL);
  	pqsignal(SIGTTOU, SIG_DFL);
  	pqsignal(SIGCONT, SIG_DFL);
  	pqsignal(SIGWINCH, SIG_DFL);
  
  	/* We allow SIGQUIT (quickdie) at all times */
  	sigdelset(&BlockSig, SIGQUIT);
  
  	/*
  	 * Create a resource owner to keep track of our resources (not clear that
  	 * we need this, but may as well have one).
  	 */
  	CurrentResourceOwner = ResourceOwnerCreate(NULL, "Wal Writer");
  
  	/*
  	 * Create a memory context that we will do all our work in.  We do this so
  	 * that we can reset the context during error recovery and thereby avoid
  	 * possible memory leaks.  Formerly this code just ran in
  	 * TopMemoryContext, but resetting that would be a really bad idea.
  	 */
  	walwriter_context = AllocSetContextCreate(TopMemoryContext,
  											  "Wal Writer",
  											  ALLOCSET_DEFAULT_MINSIZE,
  											  ALLOCSET_DEFAULT_INITSIZE,
  											  ALLOCSET_DEFAULT_MAXSIZE);
  	MemoryContextSwitchTo(walwriter_context);
  
  	/*
  	 * If an exception is encountered, processing resumes here.
  	 *
  	 * This code is heavily based on bgwriter.c, q.v.
  	 */
  	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
  	{
  		/* Since not using PG_TRY, must reset error stack by hand */
  		error_context_stack = NULL;
  
  		/* Prevent interrupts while cleaning up */
  		HOLD_INTERRUPTS();
  
  		/* Report the error to the server log */
  		EmitErrorReport();
  
  		/*
  		 * These operations are really just a minimal subset of
  		 * AbortTransaction().	We don't have very many resources to worry
  		 * about in walwriter, but we do have LWLocks, and perhaps buffers?
  		 */
  		LWLockReleaseAll();
  		AbortBufferIO();
  		UnlockBuffers();
  		/* buffer pins are released here: */
  		ResourceOwnerRelease(CurrentResourceOwner,
  							 RESOURCE_RELEASE_BEFORE_LOCKS,
  							 false, true);
  		/* we needn't bother with the other ResourceOwnerRelease phases */
  		AtEOXact_Buffers(false);
  		AtEOXact_Files();
  		AtEOXact_HashTables(false);
  
  		/*
  		 * Now return to normal top-level context and clear ErrorContext for
  		 * next time.
  		 */
  		MemoryContextSwitchTo(walwriter_context);
  		FlushErrorState();
  
  		/* Flush any leaked data in the top-level context */
  		MemoryContextResetAndDeleteChildren(walwriter_context);
  
  		/* Now we can allow interrupts again */
  		RESUME_INTERRUPTS();
  
  		/*
  		 * Sleep at least 1 second after any error.  A write error is likely
  		 * to be repeated, and we don't want to be filling the error logs as
  		 * fast as we can.
  		 */
  		pg_usleep(1000000L);
  
  		/*
  		 * Close all open files after any error.  This is helpful on Windows,
  		 * where holding deleted files open causes various strange errors.
  		 * It's not clear we need it elsewhere, but shouldn't hurt.
  		 */
  		smgrcloseall();
  	}
  
  	/* We can now handle ereport(ERROR) */
  	PG_exception_stack = &local_sigjmp_buf;
  
  	/*
  	 * Unblock signals (they were blocked when the postmaster forked us)
  	 */
  	PG_SETMASK(&UnBlockSig);
  
  	/*
  	 * Reset hibernation state after any error.
  	 */
  	left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
  	hibernating = false;
  	SetWalWriterSleeping(false);
  
  	/*
  	 * Advertise our latch that backends can use to wake us up while we're
  	 * sleeping.
  	 */
  	ProcGlobal->walwriterLatch = &MyProc->procLatch;
  
  	/*
  	 * Loop forever
  	 */
  	for (;;)
  	{
  		long		cur_timeout;
  		int			rc;
  
  		/*
  		 * Advertise whether we might hibernate in this cycle.	We do this
  		 * before resetting the latch to ensure that any async commits will
  		 * see the flag set if they might possibly need to wake us up, and
  		 * that we won't miss any signal they send us.  (If we discover work
  		 * to do in the last cycle before we would hibernate, the global flag
  		 * will be set unnecessarily, but little harm is done.)  But avoid
  		 * touching the global flag if it doesn't need to change.
  		 */
  		if (hibernating != (left_till_hibernate <= 1))
  		{
  			hibernating = (left_till_hibernate <= 1);
  			SetWalWriterSleeping(hibernating);
  		}
  
  		/* Clear any already-pending wakeups */
  		ResetLatch(&MyProc->procLatch);
  
  		/*
  		 * Process any requests or signals received recently.
  		 */
  		if (got_SIGHUP)
  		{
  			got_SIGHUP = false;
  			ProcessConfigFile(PGC_SIGHUP);
  		}
  		if (shutdown_requested)
  		{
  			/* Normal exit from the walwriter is here */
  			proc_exit(0);		/* done */
  		}
  
  		/*
  		 * Do what we're here for; then, if XLogBackgroundFlush() found useful
  		 * work to do, reset hibernation counter.
  		 */
  		if (XLogBackgroundFlush())
  			left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
  		else if (left_till_hibernate > 0)
  			left_till_hibernate--;
  
  		/*
  		 * Sleep until we are signaled or WalWriterDelay has elapsed.  If we
  		 * haven't done anything useful for quite some time, lengthen the
  		 * sleep time so as to reduce the server's idle power consumption.
  		 */
  		if (left_till_hibernate > 0)
  			cur_timeout = WalWriterDelay;		/* in ms */
  		else
  			cur_timeout = WalWriterDelay * HIBERNATE_FACTOR;
  
  		rc = WaitLatch(&MyProc->procLatch,
  					   WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
  					   cur_timeout);
  
  		/*
  		 * Emergency bailout if postmaster has died.  This is to avoid the
  		 * necessity for manual cleanup of all postmaster children.
  		 */
  		if (rc & WL_POSTMASTER_DEATH)
  			exit(1);
  	}
  }
  
  
  /* --------------------------------
   *		signal handler routines
   * --------------------------------
   */
  
  /*
   * wal_quickdie() occurs when signalled SIGQUIT by the postmaster.
   *
   * Some backend has bought the farm,
   * so we need to stop what we're doing and exit.
   */
  static void
  wal_quickdie(SIGNAL_ARGS)
  {
  	PG_SETMASK(&BlockSig);
  
  	/*
  	 * We DO NOT want to run proc_exit() callbacks -- we're here because
  	 * shared memory may be corrupted, so we don't want to try to clean up our
  	 * transaction.  Just nail the windows shut and get out of town.  Now that
  	 * there's an atexit callback to prevent third-party code from breaking
  	 * things by calling exit() directly, we have to reset the callbacks
  	 * explicitly to make this work as intended.
  	 */
  	on_exit_reset();
  
  	/*
  	 * Note we do exit(2) not exit(0).	This is to force the postmaster into a
  	 * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
  	 * backend.  This is necessary precisely because we don't clean up our
  	 * shared memory state.  (The "dead man switch" mechanism in pmsignal.c
  	 * should ensure the postmaster sees this as a crash, too, but no harm in
  	 * being doubly sure.)
  	 */
  	exit(2);
  }
  
  /* SIGHUP: set flag to re-read config file at next convenient time */
  static void
  WalSigHupHandler(SIGNAL_ARGS)
  {
  	int			save_errno = errno;
  
  	got_SIGHUP = true;
  	if (MyProc)
  		SetLatch(&MyProc->procLatch);
  
  	errno = save_errno;
  }
  
  /* SIGTERM: set flag to exit normally */
  static void
  WalShutdownHandler(SIGNAL_ARGS)
  {
  	int			save_errno = errno;
  
  	shutdown_requested = true;
  	if (MyProc)
  		SetLatch(&MyProc->procLatch);
  
  	errno = save_errno;
  }
  
  /* SIGUSR1: used for latch wakeups */
  static void
  walwriter_sigusr1_handler(SIGNAL_ARGS)
  {
  	int			save_errno = errno;
  
  	latch_sigusr1_handler();
  
  	errno = save_errno;
  }
  